By way of background, reference is made to the D. L McKinley U.S. Pat. No. 3,439,474 (1969) and U.S. Pat. No. 3,350,845 (1967) and to a publication entitled “Hydrogen permeability of Palladium-Copper membranes over a wide range of temperatures and pressures” by B. H. Howard, R. P. Killmeyer, M. V. Ciocco, B. D. Morreale, R. M. Enick and F. Bustamante, Conference proceedings of the National Hydrogen Association's 14th annual U.S. Hydrogen Conference in Washington D.C., March, 2003 all incorporated herein by reference. The “474” Patent relates to hydrogen diffusion barrier foils (herein also called membranes) “consisting essentially of an alloy of palladium and from about 30 to 60 weight percent copper, said alloy having a metallographic crystalline structure which is at least partially body-centered . . . ”. (Col. 3, lines 41–45); and it describes in detail the advantage, for achieving the highest hydrogen permeability, of operating the alloy (Col. 6, lines 22–24), in the ordered metallographic body-centered crystalline alloy structure (also commonly called bcc) over the face-centered crystalline structure (also commonly called fcc). The above referenced Howard publication further verifies the speculations put forth in the McKinley “474” patent, and additionally states, applicable references in parentheses, that “Pd—Cu alloys have exhibited resistance to sulfur poisoning upon exposure to H2-rich retentate streams containing H2S levels of up to 5 ppm [9]., 1000 ppm [10] and (at temperatures greater than 773K) intermittent exposure to 10,000 ppm [7].” (Page 2, 1st paragraph, next-to-last sentence).
Accordingly, the alloys of the present invention, later described, when limited to operate at a controlled temperature in the preferred range from about 500° C. (773K) to 600° C., are resistant to sulfur-contaminated fossil fuel reformates, including, but not limited to, steam reformed or partially oxidized diesel fuel or partially oxidized or gasified coal.
The simultaneous permeation of pure hydrogen through palladium alloy (including Pd/Cu) membranes during steam reforming of fossil fuels, including hydrocarbons, has the advantage of simultaneously producing high purity hydrogen in a single reaction step. Further, in contrast to conventional reforming at higher temperatures of the order of about 800° C., a more moderate reforming high temperature of 500° C. to 600° C. actually suffices in the important cases of simultaneously reacting steam with a hydrocarbon and permeating pure hydrogen therefrom through a palladium copper alloy membrane.
I have found that during my testing of palladium copper alloy membranes in the selected temperature range from about 500° C. to 600° C.—particularly those alloys later described for the purposes of the present invention—, that leakage through the membranes rapidly occurs which is apparently induced by the high temperature. More specifically and as outlined in the examples below, I have found that the commonly employed 60 weight % Pd/40 weight % Cu alloy (herein 60Pd/40Cu) can only be reliably operated to a maximum temperature in the range from 450° C.–500° C., but that above those temperatures, leakage to impurities rapidly develops and increases in magnitude as the membrane is cycled between the safe temperature range below about 500° C. and this newly determined damaging temperature range above 500° C. Thus it follows, for example, that in order to maintain the 60Pd/40Cu membrane leak free, it must be maintained at a temperature below about 500° C. and preferably at or below about 450° C.
Under the presumption that the observed temperature induced leakage breakage occurs due to the phase change of the 60Pd/40Cu alloy from the fully bcc phase to a full or partial fcc phase, further experiments were undertaken with different Pd/Cu alloy compositions which the above referenced “474” patent teaches to maintain a bcc phase at higher temperatures than the 60Pd/40Cu alloy of the prior art. It was thus determined that Pd/Cu alloys with slightly higher than 40 weight percent of copper not only exhibited higher damaging temperatures (said temperature which depends on the exact alloy composition) as speculated, but that, in a very critical range of composition and operating temperature range, the alloys of the present invention exhibit higher hydrogen permeability than the 60Pd/40Cu alloy. Thus I have discovered a superior group of Pd/Cu alloys when compared to the 60Pd/40Cu, specifically with a critical composition of 54–58 weight % Pd with the balance substantially Cu. This group of alloy compositions can be safely operated in a temperature range between about 500° C. and about 600° C. (depending on the exact alloy) without leakage breakage and in the range from above about 550° C. to about 600° C. wherein the alloy maintains a higher hydrogen permeability rate than the prior art 60Pd/40Cu alloy which itself develops leakage breakage over this range of temperatures. This finding is surprising in the view of the prior teaching in the McKinley “474” patent showing the 60Pd/40Cu alloy as having the highest permeability amongst the family of Pd/Cu alloys in the 30–60 weight percent copper range at 350° C.
In accordance with the discoveries underlying the present invention, it is thus found that palladium-copper alloy membranes can be maintained leak-free by carefully controlling the temperature that the membrane is subjected to during operation to a composition dependent value between about 500° C. and 600° C. The exact maximum “damaging” temperature, moreover, is dependent on the specific alloy composition in question, and generally occurs at some point in the region where hydrogen permeation decreases with increasing temperature as shown in FIG. 1. Such damaging temperatures can be pre-determined by thermal cycling and intermittent measurement of inert gas (such as helium or argon) permeation rates, by following procedures described in the examples below. It is further found that there exist slightly but critically different Pd/Cu alloys that remarkably achieve substantially constant hydrogen flux permeability at fluctuating temperatures over the 470° C. to almost 600° C. temperature range, instead of the rapidly declining hydrogen flux permeability characteristic of McKinley's 60Pd/40Cu alloy above 450–470° C.—and that they achieve such constant permeability irrespective of temperature use in such a range without suffering the gradually increasing leak of impurities into the permeated hydrogen of the prior art ratio alloys in fossil fuel reformate generation of pure hydrogen, as the temperature is increased or decreased within such range.
The invention is primarily directed to this novel performance over prior art recommended 60Pd/40Cu alloys, as will later be detailed.